The present invention relates generally to actuators for use in cyclotrons, and more particularly to actuators for use in cyclotrons utilizing a Shape Memory Alloy.
A cyclotron is a type of particle accelerator, which is used to accelerate charged particles (e.g., electrons, protons, alpha particles) up to high speed, thereby creating a beam or stream of charged particles. This beam may then be directed at a target made of a given material (e.g., H218O water or 18O2 gas) to produce particle-to-atom collisions in order to create different atoms (e.g., 18F2 gas), ions (e.g., 18F−) or other particles (e.g., alpha particles). These resulting atoms, ions or particles may then be put to various uses in research or medicine, such as for diagnostic imaging (e.g., positron emission tomography (PET), single photon emission computed tomography (SPECT), etc.) or radiation therapy (e.g., using alpha particles of electrons).
FIGS. 1 and 2 illustrate a conventional cyclotron 10, comprising two opposed “dees” 12/14 situated within a uniform magnetic field 16 created by two opposing magnets 18/20. The dees 12/14 (so called because of their “D” shape) are placed back-to-back with their straight sides 22/24 parallel to one another, but slightly separated in order to form a gap 26 between them. The dees 12/14 are contained within a vacuum field 36 bounded by a vacuum envelope or barrier 38 (which is defined by the interior surface of the pressure vessel 39 that contains the dees). The dees 12/14 are also connected to a radio-frequency (RF) voltage oscillator 28 that applies a rapidly oscillating voltage to the two dees 12/14 such that their polarities oscillate in a rapid and controlled manner. This produces an electric field 29 across the gap 26. Charged particles are injected into the magnetic field region of the first dee 12 at an injection point 30, and the beam of particles bends in a circular, constant-speed path 32 due to the influence of the magnetic field. Once the beam exits the edge 22 of the first dee 12, it continues in a straight path across the gap 26 and accelerates due to the electric field 29 in the gap 26 between the dees 12/14. The accelerated beam then crosses the edge 24 of the second dee 14 and again curves in a constant (but now higher) speed circular path (now also having a larger radius of curvature than before), until it exits the edge 24 of the second dee 14. The particle beam now accelerates in a straight line across the gap 26 again until it crosses the edge 22 of the first dee 12, and the cycle continues. As this process continues, the beam traces out a generally spiral path, getting faster and further from the center of the cyclotron on each successive loop, until it finally exits one of the dees and collides with a target 34.
While the cyclotron 10 is being operated, the magnets 18/20 may need to be monitored and regulated in order to control the magnetic field, and the RF voltage oscillator 28 may also need to be monitored and regulated in order to control the rapidly oscillating electric field. The reason these magnetic and electric fields need to be controlled is to produce a particle beam 32 in an efficient and effective manner. One common approach toward understanding how the beam is behaving is to interrupt the beam from time to time with a probe 40. There are a variety of different types of probes (such as current probes, CCD cameras, deflectors, foil strippers/extraction devices, etc.) which are useful for directly measuring or sensing various beam characteristics, interrupting or deflecting/perturbing the beam so that other devices can measure or sense various beam characteristics, interrupting the beam and stripping away electrons, etc. FIGS. 3 and 4 illustrate one exemplary approach to probe usage in a cyclotron. The probe 40 may be mounted on a shaft 42 which is rotatably supported by one or more supports 44 (shown here fastened to the floor 46 of the cyclotron chamber by two bolts 48). The shaft 42 may be turned by a stepper motor 56 as illustrated in FIG. 3. The probe 40 is typically positioned in either of two positions or orientations: a first standby position 50 in which the probe 40 does not substantially interrupt the beam path 32 (or is substantially parallel with the path 32), and a second operating position 52 in which the probe 40 does interrupt the beam path 32 (or is oriented such that its incident surface 54 is substantially normal or perpendicular to the beam path 32). The two positions 50/52 of the probe 40 may be achieved by a simple rotation of the probe shaft 42 through use of the stepper motor 56. Alternatives to the use of a rotating probe shaft 42 for placing the probe 40 into and out of the beam path 32 include the use of drive screws, trains, slides, linkages and other mechanisms, for causing the probe 40 to be telescoped toward/away from the cyclotron center (i.e., at different radii), rotated into/out of the beam path 32, etc.
In prior art approaches, the rotating shaft 42 or other mechanism for positioning the probe 40 into the beam path 32 requires the use of one or more feed-throughs 58. A feed-through 58 is a structural arrangement that allows one or more components—such as a probe-positioning mechanism, electrical power or signal wires, pneumatic or hydraulic lines, etc.—to be fed through the vacuum envelope 38. The feed-through 58 may comprise an appropriately sized hole in the pressure vessel 39 which is plugged with a vacuum-tight plug 59 through which the probe-positioning shaft 42 and/or other components pass. The pressure vessel 39 is typically made of metal, while the feed-through plug 59 may be made from a variety of materials such as high-density plastics, ceramics, metals, composites, etc. As shown in FIG. 5, a probe-positioning shaft 42 may pass through the wall of the pressure vessel 39, with a plug 59 sealing the hole in the vessel wall 39. In this example, the plug 39 divides the shaft 42 into one portion 42i which is inside the vacuum field 36 and another portion 42e which is external. The plug 59 not only provides a vacuum-tight seal, but may also provide a cylindrical internal bearing surface against which the shaft 42 or other positioning mechanism may be rotated or translated while maintaining the seal.
However, when utilizing feed-throughs 58 it is often difficult to prevent leaks and maintain an appropriate vacuum within the cyclotron chamber. This is especially true when the component passing through the feed-through is a mechanical moving member, such as a probe-positioning shaft, drive screw, train, slide, linkage or other mechanism as described above and known in the art. Additionally, it is typically not practical to place the stepper motor 56 (or other prior art devices for moving the probe 40 into position) inside the vacuum field 36 (rather than outside as illustrated in FIG. 3), due to electromagnetic interference that may be caused between the stepper motor 56 and the beam 32. It would be desirable, therefore, to provide a solution for moving a probe 40 into position inside the cyclotron's vacuum field 36 which overcomes these shortcomings